SELECTED DISCUSSIONS TO exercises/problems
BELOW.
YOU ARE RESPONSIBLE FOR ALL PROBLEMS EVEN THE ONES WITHOUT DISCUSSIONS, WHICH ARE
DESIGNED TO HELP YOU DO RELATED EXERCISES. Each sublist is separated by
a semi-colon ";" and ends with the name of the section for those
exercises. WE WILL REVIEW THEM IN-CLASS THE LAST WEEK BEFORE FINAL
EXAM.

9. Since the vector V is in the y
direction and vector B lies in the x-y plane the force points in the be
negative z direction, ( along your right thumb) wrap the velocity
into the magnetic field vector with your right
fingers. See equation 20.1; the angle between vector V and field B
is shown in diagram. Force = mass *acceleration.

16. See section 20.3.
One-quarter of full circle circumference = pi*R/2 =
0.0118 m allows you to get R. See equation 20.4; from that
relation you can get B.

31. EQUATION 20.7. The force on segment ab
is IN, by wrapping right ward I into downward B; the
force on segment cd is in the opposite direction, OUT, with the
same magnitude as the force on ab.. The force on segments bc
and da are ZERO since the currents are parallel to the magnetic
field. NOTE THE TWO NON-ZERO FORCE PRODUCE A TORQUE, CAUSING LOOP TO
ROTATE LIKE AN ELECTRIC MOTOR.

78. (a) The weight is DOWN, so the
magnetic FORCE is UP. THE CURRENT MUST FLOW RIGHTWARD
through the bottom bar. Thus, terminal a is at the higher
voltage. (b) force magnitude = ILB =mg, where I = V/R and m is the
maximum force.

34. SEE PROBLEM 31; FORCE on segment bc
and da are zero since wire segments are parallel and anti-parallel to
the field. The only
force exerting a torque about the hinge is on segment cd. THAT FORCE IS
IS OUT. Torque = (0.350 m)*force, where force ILB.

50. (a) Let us look at point Q; The upward
current causes an outward B field and the rightward current causes an
outward field. Use equation 20.10; since the fields are in the same
outward direction, just add them.
AT P, the upward current causes an inward field, and the rightward
current also causes an inward field; Again, use equation 20.10; since
the fields are in the same inward direction, just add them.
(b) Use the same method as in part (a); but you may not
simply add them to get the correct answer since the field
directions may be different this time around.